Surface ocean will change colour by century end: Study

More than 50 per cent of the world's oceans will shift in colour due to climate change by the year 2100, an MIT study has found.

Climate change is causing significant changes to phytoplankton in the world's oceans, and over the coming decades these changes will affect the ocean's colour, intensifying its blue regions and its green ones, according to the study published in the journal Nature Communications.

Satellites should detect these changes in hue, providing early warning of wide-scale changes to marine ecosystems, said researchers at the Massachusetts Institute of Technology (MIT) in the US.

They developed a global model that simulates the growth and interaction of different species of phytoplankton, or algae, and how the mix of species in various locations will change as temperatures rise around the world.

The researchers also simulated the way phytoplankton absorb and reflect light, and how the ocean's colour changes as global warming affects the makeup of phytoplankton communities.

They ran the model through the end of the 21st century and found that, by the year 2100, more than 50 per cent of the world's oceans will shift in colour, due to climate change.

The study suggests that blue regions, such as the subtropics, will become even more blue, reflecting even less phytoplankton -- and life in general -- in those waters, compared with today.

Some regions that are greener today, such as near the poles, may turn even deeper green, as warmer temperatures brew up larger blooms of more diverse phytoplankton.

"The model suggests the changes won't appear huge to the naked eye, and the ocean will still look like it has blue regions in the subtropics and greener regions near the equator and poles," said Stephanie Dutkiewicz, a principal research scientist at MIT.

"That basic pattern will still be there. But it will be enough different that it will affect the rest of the food web that phytoplankton supports," Dutkiewicz said.

The ocean's colour depends on how sunlight interacts with whatever is in the water. Water molecules alone absorb almost all sunlight except for the blue part of the spectrum, which is reflected back out, researchers said.

Relatively barren open-ocean regions appear as deep blue from space. If there are any organisms in the ocean, they can absorb and reflect different wavelengths of light, depending on their individual properties, they said.

Phytoplankton, for instance, contain chlorophyll, a pigment which absorbs mostly in the blue portions of sunlight to produce carbon for photosynthesis, and less in the green portions.

As a result, more green light is reflected back out of the ocean, giving algae-rich regions a greenish hue, researchers said.

Dutkiewicz said chlorophyll does not necessarily have to reflect the sensitive signal of climate change.

Any significant swings in chlorophyll could very well be due to global warming, but they could also be due to "natural variability" -- normal, periodic upticks in chlorophyll due to natural, weather-related phenomena.

"An El Nino or La Nina event will throw up a very large change in chlorophyll because it is changing the amount of nutrients that are coming into the system," Dutkiewicz said.